MYH1 mutation

"The frequency of the MYH1 mutation in the general Quarter Horse population is about 4%. About 7.5% of Quarter Horses have 1 copy of the mutation. The mutation frequency is higher in the reining (13.5%), working cow (8.5%) and halter (8%) categories, and not observed in barrel and racing categories."

"Quarter Horse and related breeds are susceptible to developing rapid onset of muscle atrophy and severe muscle damage at rest (nonexertional rhabdomyolysis). An autoimmune muscle disease called immune-mediated myositis (IMM) can cause this severe atrophy, which can result in the loss of 40% of muscle mass within 72 hours in Quarter Horse and related breeds. IMM is characterized by infiltration of inflammatory cells, particularly lymphocytes, into muscle fibers and surrounding blood vessels, with preferential targeting of the gluteal (rump) and epaxial (along the vertebral column) muscles. IMM is characterized by stiffness, weakness and nonspecific malaise. Affected horses are usually 8 years and younger or 17 years and older, with no sex predilection. "

"The frequency of the MYH1 mutation in the general Quarter Horse population is about 4%. About 7.5% of Quarter Horses have 1 copy of the mutation. The mutation frequency is higher in the reining (13.5%), working cow (8.5%) and halter (8%) categories, and not observed in barrel and racing categories."

"Quarter Horse and related breeds are susceptible to developing rapid onset of muscle atrophy and severe muscle damage at rest (nonexertional rhabdomyolysis). An autoimmune muscle disease called immune-mediated myositis (IMM) can cause this severe atrophy, which can result in the loss of 40% of muscle mass within 72 hours in Quarter Horse and related breeds. IMM is characterized by infiltration of inflammatory cells, particularly lymphocytes, into muscle fibers and surrounding blood vessels, with preferential targeting of the gluteal (rump) and epaxial (along the vertebral column) muscles. IMM is characterized by stiffness, weakness and nonspecific malaise. Affected horses are usually 8 years and younger or 17 years and older, with no sex predilection. "

How is it that there seem to so many genetical diseases in the AQHA breed? Probably due to more extensive testing so that simply more are known than in, say, arabians, tbs or wbs?
With as many horses as there are of the breed, it cannot be a narrow gene pool...can it?

I suspect it is partly a numbers game - 4% of a gazillion quarter horses is enough horses to be noticeable as a trend and should be looked into, while 4% in a breed with the population in the thousands is more likely to be passed off as freak issues, especially when the quarter horses are fairly concentrated in one country.

Probably at this point they don't want to point to any one line until they know more. PSSM they haven't ever said where it actually is coming from though there are suspects way back. Impressive is the only one they actually have pointed to a certain sire.